Dallas Rangemaster: voltage collector

[ricci]

Hi everyone, I finally managed to build myself the Dallas Rangemaster treble booster with negative ground and with an OC44 germanium transistor and standard values except for the 3.9K resistor which I don't have and which I replaced with a 3.5k resistor. The pedal works but on the collector of the transistor I measure 1.38 V, so very far from the 7 V required, but it works. Given that I am using a Spot one as a power source. What can it depend on ?

Thanks in advance

[iainpunk]

try replacing the 68K with a 100K trim pot!
also, what are the base and emitter voltages?

cheers, Iain

[ricci]

Vbase =8,66 V
Vemitter = 8,93 V
Vcollector= 1,43 V

Why do you suggest me to replace 68k with 100k ?

Edit:

ok ok, now I reread and I missed that you were referring to a 100k pot, in order to find the resistance you need for the bias.

[willienillie]

I think your measurements are all upside down.  9V - 1.43V = 7.57V.

[iainpunk]

I think your measurements are all upside down.  9V - 1.43V = 7.57V.

i think exactly the same!

[ricci]

I think your measurements are all upside down.  9V - 1.43V = 7.57V.

what do you mean ?

[PRR]

...all upside down. ...

One usually-authoritative site, ElectroSmash, gives it that way.

https://www.electrosmash.com/images/tech/rangemaster/dallas-rangemaster-bias-points.png
https://postimg.cc/Yvf3pycL

Which may be another reminder to say WHICH plan you are working from.

1.5V drop in Rc is not wrong.

How bad does it sound??

[Big Monk]

Hi everyone, I finally managed to build myself the Dallas Rangemaster treble booster with negative ground and with an OC44 germanium transistor and standard values except for the 3.9K resistor which I don't have and which I replaced with a 3.5k resistor. The pedal works but on the collector of the transistor I measure 1.38 V, so very far from the 7 V required, but it works. Given that I am using a Spot one as a power source. What can it depend on ?

Thanks in advance

How are you achieving negative ground but using the OC44?

[ricci]

How are you achieving negative ground but using the OC44?

https://www.electrosmash.com/images/tech/rangemaster/dallas-rangemaster-positive-negative-ground-circuits.png

Edit: central image

[ricci]

...all upside down. ...

One usually-authoritative site, ElectroSmash, gives it that way.

https://www.electrosmash.com/images/tech/rangemaster/dallas-rangemaster-bias-points.png
https://postimg.cc/Yvf3pycL

Which may be another reminder to say WHICH plan you are working from.

1.5V drop in Rc is not wrong.

How bad does it sound??

I will check the positioning of the multimeter better. The sound is ok.

[ricci]

Looking at the survey posted by PRR, i repeated the measurement and found that the voltage collector is -8.44V with respect to + 9V and 1.56V with respect to ground. Tell me if that's okay.

[antonis]

Arithmetically, it's wrong..
(8.44 + 1.56 result into 10V..)

Measure your power supply voltage..

[ricci]

I noticed it and it is also curious because the output voltage from my power supply is 9.50 V. I don't know what to say. Does perpetual motion exist?

[Steben]

 

[PRR]

For a "clean" amp, drawn this way, we might like 4V across the collector resistor for equal swing both ways.

For a "distortion" amp we offset it so sonic damage happens sooner and happens longer. 2V, 1.5V, maybe little difference.

To play, add a 50k or 100k trim pot in series with that 68k resistor and turn it.

[ricci]

I soldered a 100k pot instead of 68K (not in series with this one) and a 10k pot instead of 3.9k but I get at most -5.60V, putting the black tip on the collector and the red one on -9V. But is there a way to fix the bias by ear?

[mozz]

Positive ground circuit. Red goes to ground. Measure collector voltage with other lead. Set 3.9k. Adjust 100k pot.

EDIT: now i see you used a pnp but use negative ground. I have to ask why as there are npn germaniums around.

[ricci]

So, correct me if I'm wrong, so...in the case of negative ground, like mine, it should be the other way around and that is that black must go to ground and red to collector. I had to invert the leads otherwise the multimeter gives random values.

[willienillie]

You mentioned in your first post that you built it negative ground with a (PNP) OC44.  There's more than one way to do that, but basically you've built it "upside down" when compared to the original, positive ground PNP circuit.  If you want to compare your collector voltage to an original that has roughly -7 volts relative to ground, you should measure between your transistor's collector and the "far end" of the emitter resistor, which in your case is supply (nominally +9V).  That's why I said "9V - 1.43V = 7.57V" above.  If your actual supply is 9.5V or 10V, it doesn't change much.

Swapping the meter leads should only change the polarity (+/-) of the readings.  It doesn't really matter, we know the polarity is correct for the way you built the circuit.  It's the |absolute value| that you're looking for.  But even that doesn't really matter either, the "target" collector voltage is just a ballpark number.  Now that you have a trimmer to adjust bias, you should set it by ear to whatever bias point sounds best to you.

[ricci]

So, if I understand correctly, what matters is whether you like the sound regardless of the value of the resistors.